Observations of Enhanced Diapycnal Mixing near the Hawaiian Ridge

Abstract

Profiles of potential density obtained from CTD casts at two stations at different distances from the Hawaiian ridge are examined for evidence of diapycnal turbulent mixing as indicated by density inversions and internal-wave vertical strain. Results from independent casts are used to produce ensemble-averaged vertical distributions for the number of inversions and the Thorpe scale. Both parameters were found to be higher over the slope of the topography at 2500-m depth than in the deep ocean, 110 km to the north. Thorpe scale–based estimates of the rate of dissipation of turbulent kinetic energy and turbulent vertical diffusivity are elevated by an order of magnitude over the slope relative to deep ocean background levels. The vertical distributions of these mixing parameters are nonuniform and exhibit signs of locally enhanced dissipation, possibly due to internal tides generated at the ridge. At the deep station, turbulence is at background levels from the surface down to 2000 m. Below this, a localized zone of enhanced mixing is observed, within which the dissipation rate is O(10−9 W kg−1) and turbulent diffusivity is greater than O(10−4 m2 s−1), perhaps due to an internal tide ray originating at the ridge. The full-depth topographical enhancement of mixing near the ridge also appears in the vertical strain field. Estimates of dissipation rate and turbulent diffusivity, based on an internal wave–wave interaction model, give results similar to direct Thorpe scale methods, except in weakly stratified environments where both methods are subject to uncertainty. Near the topography, the variation in mixing intensity observed between casts is sensitive to sporadic large mixing events, which are triggered by internal waves associated with the spring tide. The upper portion of the water column (stronger stratification) is more responsive to the tide than the deep regions.